The rising cost and short supply of fossil fuels have stimulated interest in alternate sources of energy for space heating and cooling. Heating a structure with solar energy is not new. Embodiments of this idea have been conceived and put to use well before this century. The intermittent nature of solar radiation, however, requires either an auxiliary heating system or a method of storing solar heat energy for later use.
One of parameters parametes of a solar energy system is the size of the heat storage tank. In energy systems using water, the parameter is the size of the water tank where solar energy is stored in the form of heat. The size of the tank affects the rate of heat absorption and this rate is in turn dependent upon available solar radiation. Various methods of heat storage have been utilized with limited success. A solar heating system using an underground tank is shown by Cornwall in U.S. Pat. No. 2,553,302. Systems of underground pipes and lines to store heat are shown by Gay in U.S. Pat. No. 2,584,573 and Hervey in U.S. Pat. No. 3,339,629. Tanks of water and water lines in beds of rock or gravel will readily store enough heat for 1 or 2 days. Larger capacity tanks and lines are not economically feasible. The principal feature of these closed systems is an expansive underground network of piping or tubing and a heat transfer fluid which is circulated through the piping in order to add heat to or remove heat from the ground. The underground network is costly to install and would be difficult to repair in case of leakage or plugging up of flow underground.